The present invention relates to a diode bolometer and to a method for producing a diode bolometer and, in particular, to a diode bolometer exhibiting reduced thermal coupling and to a method for producing of high alignment tolerance.
Detecting infrared radiation is increasing in importance in many different fields. For the automobile industry, this importance is achieving an increase in the security of, for example, pedestrians who can be made visible using infrared sensors also in dark environments. When coupling an automatic brake system to sensor technology, accidents can be avoided or at least the consequences thereof be reduced. Further applications of infrared sensors exemplarily include examining technical apparatuses (such as, for example, electrical lines or circuit boards) or buildings. Medical applications may also become relevant in the future. Infrared sensors are already employed in the field of monitoring buildings, areas and frontiers.
The achievable resolution of minimum temperature differences is an important criterion for the quality of the measuring instrument used in many of these applications. This sensitivity is, in commercial apparatuses, frequently indicated as NETD (noise equivalent temperature difference), wherein values of, for example, below 100 mK in temperature difference are achieved in uncooled bolometers. The designation of this characteristic quantity also explains the internal limits of sensors given by the noise features of the system used. When, for example as detector material, a thin membrane is used as a sensor, the membrane heating up under the influence of infrared radiation and thus altering its electrical resistance, the electrical features of said system determine which changes in resistance (and thus temperature) can still be detected and separated from the noise background. When the alteration in the resistance of the material induced by the change in temperature is smaller than the noise of the electrical parameters, it will no longer be resolved.
In many homogenous amorphous sensor materials (such as, for example, silicon, vanadium oxide, etc.), the percentage change in resistance is proportional to the change in temperature. The constant of proportionality here is basically determined by the material selected and the process parameters, wherein generally there are tight limits for optimization. Typical values of the change in resistance are in a range of roughly 2 to 3% per K.
When the change in resistance is determined by the features of the sensor material, two other basic ways of acting on the sensor features on a larger scale remain. A first way is making the sensor elements to be as large as possible. The greater the area available for the sensor and the thermal insulation regions thereof, the more radiation can be absorbed and the more radiation energy will be transformed to an increase in temperature of the sensor. However, this approach is of the decisive disadvantage that the increasing demand for miniaturization and, thus, making the devices cheaper, cannot be taken into account.
When the aim is optimizing the signal-to-noise ratio with a constant size for cost reasons, noise minimization remains as another approach. In electronic devices, there are different noise sources. In amorphous materials, the so-called 1/f noise, where the noise power density is inversely proportional to the frequency f, is usually predominant. This represents a serious problem in that the integrative read-out circuits conventionally used (low pass), for example, are not suitable for suppressing predominant low-frequency noise portions.
A way of bypassing this problem is using a single-crystalline material, such as, for example, silicon. In materials of this kind, the 1/f noise is usually not predominant and a good signal-to-noise ratio can be achieved by integrating the measuring signal. However, this advantage usually entails a greatly reduced dependence of the resistance on the temperature. The temperature dependence of the resistance value may exemplarily have a value of 0.3% per K.
For this reason, it may also be of advantage to use single-crystalline diodes as infrared sensors. An ideal diode exhibits an exponential temperature dependence of the resistance and noise is determined by shot noise which can be limited by a suitable integration time.
Shot noise essentially depends only on the frequency bandwidth of the measurement and a 1/f2 dependence only becomes evident with higher frequencies. Thus, diodes are very promising sensor elements since they are able to combine a great signal and low noise.
However, integrating thermally insulating diodes in a CMOS process is complicated. The approach used at first of manufacturing insulated diodes directly in the CMOS wafer by means of suitable under-etchings is of disadvantage in that it consumes large areas—without combining useful insulation and absorption features.
These techniques, for example, do not allow integrating active CMOS driving and amplifying elements below the sensor structure. The potential integration of these elements laterally beside the detector results in a marked reduction of the filling factor (detector area relative to total area) and, thereby, increases the chip area and detector costs. Additionally, it is not possible to produce, using these structures, an absorber comprising a resistance layer having a layer or sheet resistance of roughly 377 Ohm/□ (approximate spreading resistance of an electromagnetic wave in air) and being at the same time arranged at an altitude of λ/4 (i.e. approx. 2.5 μM with a preferred IR wavelength λ) above a reflector.